The use of interferometry to measure changes in position, length, distance or optical length is well known. Rapidly increasing demands and needs for higher accuracy determinations of the precise time at which such interferometric measurements are taken have fueled numerous efforts to reduce and minimize the various sources of uncertainty that are inherent in currently known methods and apparatus for calculating position measurement times. These prior art procedures have primarily focussed on carefully controlling the time measurement delays so as to enable reliance upon predetermined extrapolation constants with which the actual time measurements taken are modified or corrected so as to achieve apparently enhanced accuracy. Such procedures, however, are complex and expensive and are themselves subject to external, substantially uncontrolled forces such, for example, as wear of mechanical parts and age and thermally-induced operational drift in the measurement electronics.